Add GetSequenceNumber function to PPB_Flash_Clipboard

ppapi/cpp/private/flash_clipboard.h

Index: ppapi/cpp/private/flash_clipboard.h
diff --git a/ppapi/cpp/private/flash_clipboard.h b/ppapi/cpp/private/flash_clipboard.h
index aab06efb1e91a67f5f81fb392c2d47121ce09510..0df6028cf5a35b5f6e1da8d7c972ab834b255346 100644
--- a/ppapi/cpp/private/flash_clipboard.h
+++ b/ppapi/cpp/private/flash_clipboard.h
@@ -45,6 +45,12 @@ class Clipboard {
                         PP_Flash_Clipboard_Type clipboard_type,
                         const std::vector<uint32_t>& formats,
                         const std::vector<Var>& data_items);
+
+  // Outputs a sequence number that uniquely identifies the clipboard state in
+  // |sequence_number| and returns true if successful.
+  static bool GetSequenceNumber(const InstanceHandle& instance,
+                                PP_Flash_Clipboard_Type clipboard_type,
+                                uint64_t* sequence_number);
 };
 
 }  // namespace flash